There are many kinds of railroad cars for carrying particulate or granular material. These materials are not liquid, yet may in some ways tend to flow in a somewhat liquid-like manner. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, by which the particulate material exits the car.
Consider, for example, a hopper car for transporting an almost powder-like material, such as flour or cement. Flour and cement tend to self-pack during motion of the car, such that the lading may not necessarily leave the car when a bottom gate is opened. One approach to the shipment and unloading of flour is to employ an air flow. Air is introduced at the gate to permeate the load, such that the load behaves like a fluidized bed, and hence is inclined to flow out the gate. Second, a jet of air is used to induce flow of the powder substance along an outflow conduit, much in the nature of an ejector pump. The powder leaving the gate is entrained in the airflow, and is carried to the desired destination. Railroad cars used for this purpose may have pressure vessel bodies. That is, the hatches seal, and, during unloading, the car may be modestly pressurized to perhaps 10 or 15 psi.
Although these cars are made of steel, the external shell may be thought of as a membrane. For example, the cars may have an overall width of 128 inches, and a wall skin thickness of roughly 3/16″, giving an aspect ratio on the order of 700:1 The length of the car may vary depending on the density of the lading for which the car is built, but, typically may be of the order of 30-40 ft between truck centers, and perhaps 40 to 50 ft over the strikers.